Calculate the molarity of a solution made by dissolving 12.5 grams of Na2CrO4 in enough water to form exactly 550 mL of solution. (b) How many moles of KBr are present in 150 mL of a 0.275 M solution? (c) How many milliters of 6.1 M HCl solution are needed to obtain 0.100 mol of HCl?

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(a) To calculate the molarity of a solution, we need to know the number of moles of the solute (Na2CrO4) and the volume of the solution in liters.

First, let's calculate the number of moles of Na2CrO4:
Molar mass of Na2CrO4 = 2(22.99 g/mol) + 1(51.9961 g/mol) + 4(15.999 g/mol) = 161.97 g/mol

Number of moles of Na2CrO4 = (12.5 g) / (161.97 g/mol) = 0.0771 mol

Next, we convert the volume of the solution to liters:
550 mL = 550 mL / 1000 mL/L = 0.55 L

Now, we can calculate the molarity (M) of the solution:
Molarity (M) = moles of solute / volume of solution in liters
M = 0.0771 mol / 0.55 L ≈ 0.14 M

Therefore, the molarity of the solution made by dissolving 12.5 grams of Na2CrO4 in 550 mL of water is approximately 0.14 M.

(b) To calculate the number of moles of KBr, we need to know the molarity (M) of the solution and the volume of the solution in liters.

Given:
Molarity (M) = 0.275 M
Volume = 150 mL = 150 mL / 1000 mL/L = 0.150 L

Moles of KBr = Molarity (M) x Volume (L)
Moles of KBr = 0.275 M x 0.150 L = 0.04125 mol

Therefore, there are approximately 0.04125 moles of KBr present in 150 mL of a 0.275 M solution.

(c) To calculate the volume of the HCl solution needed to obtain a certain number of moles of HCl, we can use the formula:

Volume (L) = Moles / Molarity (M)

Given:
Molarity (M) = 6.1 M
Moles = 0.100 mol

Volume (L) = 0.100 mol / 6.1 M ≈ 0.016 L

Since 1 L is equal to 1000 mL, the volume in milliliters is:
Volume (mL) = 0.016 L x 1000 mL/L = 16 mL

Therefore, approximately 16 mL of a 6.1 M HCl solution is needed to obtain 0.100 mol of HCl.

To calculate the molarity of a solution, we use the formula:

Molarity (M) = moles of solute / volume of solution in liters

Let's go step by step for each question:

(a) Calculate the molarity of Na2CrO4 solution:

Step 1: Convert the given mass of Na2CrO4 from grams to moles using its molar mass.
The molar mass of Na2CrO4 = (2 * atomic mass of Na) + atomic mass of Cr + (4 * atomic mass of O)
= (2 * 22.99 g/mol) + 52.00 g/mol + (4 * 16.00 g/mol)
= 161.97 g/mol

Moles of Na2CrO4 = mass / molar mass
= 12.5 g / 161.97 g/mol
= 0.077 mol

Step 2: Convert the given volume of the solution from milliliters (mL) to liters (L).
Volume of solution = 550 mL = 550 / 1000 L
= 0.55 L

Step 3: Use the formula for molarity to calculate the molarity of the Na2CrO4 solution.
Molarity (M) = moles of solute / volume of solution in liters
= 0.077 mol / 0.55 L
≈ 0.14 M

Therefore, the molarity of the Na2CrO4 solution is approximately 0.14 M.

(b) Calculate the number of moles of KBr present in the given solution:

Step 1: Multiply the molarity of the KBr solution by the volume of the solution in liters to get moles of KBr.
Moles of KBr = Molarity (M) * Volume of solution (in liters)
= 0.275 M * 0.150 L
= 0.04125 mol

Therefore, there are 0.04125 moles of KBr present in 150 mL of a 0.275 M solution.

(c) Calculate the volume of 6.1 M HCl solution needed to obtain 0.100 mol of HCl:

Step 1: Use the formula for molarity to rearrange and solve for the volume of HCl solution.
Moles of solute = Molarity (M) * Volume of solution (in liters)
0.100 mol = 6.1 M * x L (let x be the unknown volume we need to find)

Rearranging the equation:
x L = 0.100 mol / 6.1 M
x L ≈ 0.0164 L

Step 2: Convert the volume from liters to milliliters.
Volume needed = 0.0164 L * 1000
≈ 16.4 mL

Therefore, approximately 16.4 mL of the 6.1 M HCl solution is needed to obtain 0.100 mol of HCl.